1. Field of the Invention
The present invention relates to methods and apparatus for extruding bicomponent, side-by-side polymer fibers whose components have significantly different viscosities and, more particularly, to spinneret hole configurations and arrangements of components therein that prevent bending of side-by-side components of different viscosities upon extrusion.
2. Description of the Related Art
Woven and non-woven fabrics and yarns having desirable qualities can be manufactured from crimped side-by-side, bicomponent synthetic polymer fibers. Such bicomponent fibers typically include two different polymers arranged as microfilaments or segments across the transverse cross section of the fiber, which segments extend continuously along the length of the fiber. A melt spinning process involving extrusion of the molten polymer from orifices of a spinneret can be used to form these side-by-side bicomponent fibers. By causing one or both of the constituent segments to crimp after extrusion, a fine denier fabric or yarn can be produced with improved characteristics, such as greater bulkiness and softness, superior flexibility and drape, and better barrier and filtration properties for use in products such as disposable absorbent articles, medical garments, filtration materials, apparel, and carpet.
As is well known in the art, a side-by-side bicomponent or biconstituent fiber, in which the polymer components have significantly different thermal shrinkage characteristics, will form helical crimps when the fiber is subjected to heat, as described in U.S. Pat. No. 5,093,061 to Bromley et al., the disclosure of which is incorporated herein by reference in its entirety. A yarn made of side-by-side conjugate filaments will also develop crimps if the yarn is stretched slightly and then allowed to relax. A high degree of crimping of the bicomponent fibers is desirable, since a lofty or bulky non-woven fabric having good softness, flexibility and drape characteristics and barrier properties results.
Side-by-side bicomponent fibers can also be useful where one of the components is used as an adhesive to bond the fibers into a web. In this case, one component typically has a significantly lower melting temperature than the other component and, upon heating to the softening point, permits adjacent fibers to be bonded to each other without melting the other component.
At present, common methods of producing side-by-side conjugate fibers employ an arrangement that introduces two separated polymer streams, A and B, into the spinneret orifice through narrow channels from opposite directions above a spin hole. FIG. 1 illustrates a conventional spinneret 10 having one channel 12 that directs a stream of polymer A downstream and another channel 14 that directs a separate stream of polymer B downstream. Channels 12 and 14 respectively deliver polymers A and B to the upstream end of a cylindrical counterbore 16 that tapers at its downstream end to a spinneret hole 18 forming an orifice 20 at the bottom face of spinneret 10. The spinneret hole 18 has a round cross-section transverse to the flow direction, as shown in FIG. 2. The polymers form a side-by-side flow inside the spinneret hole 18 as well as through the orifice 20, with each component occupying a substantially. semi-circular transverse area within the spinneret hole. The fiber thus produced has a substantially round, side-by-side transverse cross section.
In prior art fiber melt spinning systems, the viscosities of the two side-by-side polymer components, which are a function of temperature, must be matched. If the viscosities of two polymer components are different, the higher viscosity polymer component flowing through the spinneret orifice loses more momentum than is lost by the low viscosity component This loss of momentum is due primarily to friction between the polymer and the spinneret hole wall. Consequently, at the exit orifice, the low viscosity polymer component pushes the high viscosity component transversely and causes the combined polymer extrudate to bend or deflect in the direction of the high viscosity polymer component. This bending phenomenon, shown in FIG. 3, is commonly referred to as extrudate dogleg. Extrudate with a high degree of dogleg can flow along and contact the spinneret bottom surface, causing the combined polymer components to become, in effect, un-spinnable. Therefore, matching the viscosities of two polymers at the spin pack orifices has been heretofore essential, limiting the permissible viscosity differences and, thereby, the crimp that is obtained in the bicomponent fiber. It has been observed that drawn fibers formed from certain polymers with greater viscosity differences exhibit a high degree of crimping. Thus, many desirable fibers formed of highly-crimpable polymer combinations may often be un-spinnable.
When the viscosities of the two polymers are equal at the spinning orifice, the polymer extrudate is straight and perpendicular to the downstream spinneret surface; i.e., there is no bending or dogleg. When the viscosities of the two polymers are different, the degree of bending or dogleg is determined by: the viscosity difference; the spinneret hole length (or, the ratio of spinneret hole length to spinneret hole diameter, L/D); the polymer flow rate through the orifice; and the volume flow rate ratio between the two polymers. Bending of the extrudate increases with the increase of the viscosity difference, the orifice length and the polymer flow rate; bending can be increased or decreased by varying the polymer flow rate ratio.
It is difficult to find pairs of polymers that yield the desired final spun product and also have matched viscosities for a specified range of spinning temperatures. For example, desirable polymers for forming side-by-side bicomponent fibers may include polyester (polyethylene terepthalate or PET) and polybutylene terepthalate (or PBT). Due to limited availability of commercial grades of these two polymers and other reasons (e.g., economical reasons), one must necessarily choose a PET and a PBT with slightly mismatched viscosities. Consequently, only a limited number of commercially available polymers have been usable to form crimpable side-by-side bicomponent fibers that yield fabrics and yams having the aforementioned highly desirable qualities. Accordingly, there remains a need for methods and apparatus capable of melt spinning side-by-side bicomponent fibers whose components have significantly different viscosities.
Therefore, in light of the above, and for other reasons that become apparent when the invention is fully described, an object of the present invention is to provide processes and apparatus capable of compensating for viscosity differences between melt-spinnable polymers in order to prevent excessive bending of such polymers when extruded side-by-side from orifices of a spinneret.
Another object of the present invention is to increase the number of polymer combinations available for forming side-by-side bicomponent fibers by expanding the acceptable range of polymer viscosity mismatches that will yield melt-spinnable fibers without excessive extrudate bending.
Yet another object of the present invention is to produce highly crimpable side-by-side bicomponent fibers from pairs of polymers having substantially mismatched viscosities.
Still another object of the present invention is to provide asymmetric cross-sectional geometries of side-by-side polymer streams within a spinneret hole that compensate for polymer viscosity differences, thereby preventing extrudate dogleg bending.
It is another object of the present invention to produce yarns, fabrics and textile products having improved characteristics from side-by-side bicomponent fibers whose components have mismatched viscosities.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the present invention, the aforementioned difficulties associated with extruding side-by-side polymers to form bicomponent fibers are overcome by employing a spinneret hole configuration that is asymmetric with respect to the arrangement of the component polymer streams flowing through and extruded from the spinneret hole. In particular, the components are arranged within the spinneret hole such that the ratio of the perimeter of the spinneret hole bounding the cross-sectional flow area of the lower viscosity component to the cross-sectional flow area of the lower viscosity component is greater than the corresponding ratio for the higher viscosity component. In other words, polymer viscosity differences that normally lead to dog-legging can be reduced or eliminated by arranging the polymer streams to increase the relative amount of surface area of the spinneret hole that contacts the lower viscosity component.
The transverse cross section shape of the spinneret hole and the resulting conjugate fiber may be trilobal, triangular, teardrop, bulb-and-stem or any other configuration that permits the lower viscosity component to occupy a portion of the transverse cross-section having a greater perimeter-to-area ratio and hence permits the higher viscosity component to occupy a portion of the transverse cross-section having a lesser perimeter-to-area ratio. The spinneret hole geometries and the associated asymmetric polymer component arrangements of the present invention advantageously allow heretofore un-spinnable combinations of polymers with mismatched viscosities to be successfully melt spun into crimpable side-by-side bicomponent fibers that can be formed into yarns and fabrics with superior characteristics, such as greater bulkiness and softness, superior flexibility and drape, and better barrier and filtration properties for use in products such as disposable absorbent articles, medical garments, filtration materials, apparel, and carpet.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof wherein like reference numerals in the various figures are utilized to designate like components. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.